23: Farthest, Coldest ""Planet"" Spied Well Beyond Pluto

Sedna’s feeble glow and slow motion show that it lies far beyond Pluto.

Courtesy of Mike Brown/Caltech

Each morning, planetary scientist Mike Brown sits down in his office at Caltech and sifts through a few hundred images gathered the previous night by the Samuel Oschin telescope atop Palomar Mountain in California. Most of the snapshots turn out to be duds, but on March 15 he announced that one series of them had captured something extraordinary: the farthest, coldest object in the solar system and the largest new member of the sun’s family of planetlike objects since the discovery of Pluto in 1930.

The enigmatic body, later named Sedna, didn’t look like much—a mere shifting speck of light on three sequential images—but its behavior immediately grabbed Brown’s attention. “I just stared at it. I’d never seen anything moving that slowly, and so very far away, that it was still big enough to be seen. I didn’t think it could possibly be real,” he says. Months of observation and orbit tracking with other telescopes proved that Sedna is very real and very strange.

Brown’s analysis indicates Sedna has a diameter of 800 to 1,100 miles (roughly three-fourths the size of Pluto), a surface temperature near –400 degrees Fahrenheit, and a peculiarly ruddy color. It follows an unusual orbit, swinging from 7 billion to 93 billion miles away from the sun and taking a leisurely 12,000 Earth years to complete its own year. Sedna’s looping path was likely caused by the gravitational jostling of both Neptune and a passing star.

Scientists aren’t sure what to call Sedna. In size it resembles Pluto, long classified as a planet, but its orbit is unique, more like that of a comet. For now it is one-of-a-kind, but Brown anticipates lots of company. “We could only have seen Sedna for something like 200 years out of its 12,000-year orbit, and then it would be too faint to see,” he says. “A rough calculation (12,000 divided by 200) tells us that there are something like 60 more objects at least the size of Sedna out there. Even if there are only 30, it is a safe bet at least 10 are a couple of times bigger than Pluto, and the biggest one could be the size of Mars. Eventually, we will find them.”